Laser isotope separation using an electron beam evaporation source to provide atomic vapor requires vapor flux densities in the region of the extraction of the particular isotope to be temporally uniform. In general, when electron beams are utilized for bombarding a mass of material to vaporize it, fluctuations in the electron beam cause fluctuations in the vapor flux density and, in certain cases, may prevent reaching desired vapor density levels. Variation in vapor flux density results in decreased efficiency for systems, such as laser isotope separation systems, which require the continuous production of a vapor.
By way of background, electron beam evaporation sources have been used as a convenient means for evaporating high vapor temperature material such as uranium and other refractory metal elements. Such a technique has been found to be useful for the evaporation of uranium in high quantities for production level isotope separation as discussed in U.S. Pat. Nos. 3,772,519 and 3,944,825, both assigned to the assignee hereof. As used in such apparatus, it is preferable to employ a linear electron beam evaporation source such as described in U.S. Pat. Nos. 4,058,667 and 4,000,423, both assigned to the assignee hereof, to provide a long line source of uranium vapor for subsequent isotopically selective ionization and separation. Such a vapor source is also preferably operated at a high vaporization rate for increased enrichment efficiency. In order to achieve high efficiency at high vaporization rates, it is critical that the vapor flux density be as uniform as possible.
In electron beam evaporation sources utilizing an elongated linear filament, the above-mentioned vapor flux modulations are a function of the current utilized to heat the filament. Under ordinary circumstances, 60 Hz heater current is employed. As a result, the emission current caused by the flow of electrons from a filament to a melt of vapor-generating material modulates at 120 Hz with its minima corresponding to the heater current maxima and minima. This electron beam fluctuation is caused by "trapping" of electrons near the filament by the magnetic field produced by the heater current. That is, the Larmor force on the electrons from the magnetic field encircling the filament causes some of the electrons to be driven back toward the filament. Since the electrons are driven back to the filament on a periodic basis, the intensity of the electron beam fluctuates in a periodic fashion, which in turn results in the vapor density fluctuating in a like periodic fashion. When vapor flux density modulations are measured using the background ion current, vapor flux density modulations follow the emission current modulations. In prior art systems, vapor flux density modulations have been measured as high as 50% in uranium evaporators.
While it might be thought that the vapor flux modulation problem could be solved by the use of a d.c.-heated filament, analysis shows that the lifetimes of the a.c. heated filaments are at least an order of magnitude greater than the lifetimes of corresponding d.c.-driven filaments. The use of an a.c. filament drive is exceptionally important for the longevity of laser isotope separation systems in which high evaporation rates require the filaments to deliver as much as 34 amps of emission current per meter of elongated linear filament.